Optical displacement sensor

ABSTRACT

The present invention relates to a compact and inexpensive optical displacement sensor that does not require accurate control of the distance to the object. A repetitive optical structure is utilized for formation of a repetitive optical signal emitted by an illuminated moving object. The repetitive optical structure is illuminated by the light source for formation of a fringe pattern (similar to Laser Doppler Anemometry), and/or, an object is illuminated by the light source and the repetitive optical structure diverts light from the illuminated object onto light sensors. A speckle pattern is formed on the object by the illumination. The speckle pattern moves with movement of the object, and speckle pattern movement is determined without a need for imaging the object onto the repetitive optical structure. Since the speckle pattern is not imaged onto the optical member, the distance and possible distance changes between the object and the optical member substantially do not affect system performance.

FIELD OF THE INVENTION

The present invention relates to a compact and inexpensive opticaldisplacement sensor for determination of displacement of an object, suchas linear displacement, angular displacement, vibration, etc.

BACKGROUND OF THE INVENTION

Provision of compact and inexpensive sensors for determination ofmovement of an object has been pursued for some time.

In Detection of movement with laser speckle patterns: statisticalproperties, Schnell et al., Vol. 15, No. 1, January 1998, J. Opt. Soc.Am., a sensor for determination of in-plane movement of a diffusingobject is disclosed. The object is illuminated with coherent light and aspeckle pattern is formed by interaction of the light with the surfaceof the object. Two interlaced differential comb photo detector arraysact as a periodic filter to the spatial-frequency spectrum of thespeckle pattern intensity. The detector produces a zero-offset, periodicoutput signal versus displacement that permits measurement of themovement at arbitrarily low speed. The direction of the movement can bedetected with a quadrature signal produced by a second pair ofinterlaced comb photo detector arrays.

In WO 98/53271, a sensor for determination of angular displacement ofone or more parts of an object is disclosed. The determination is alsobased on speckle patterns and is independent of the distance to theobject, any longitudinal and transversal movements, the shape of theobject, and the radius of angular displacement.

In T. Ushizaka, Y. Aizu, and T. Asakura: “Measurements of Velocity Usinga Lenticular Grating”, Appl. Phys. B 39, 97-106 (1986), and Y. Aizu:“Principles and Development of spatial Filtering Velocimetry”, Appl.Phys. B 43, 209-224 (1987), discloses utilization of a so-calledlenticular grating for deflection of light scattered from a particleonto photo detectors. A particle traversing a measurement volume isilluminated by diffuse light from a He—Ne laser (5 mW) illuminating aground glass placed at the measurement volume. It is a fundamentalcharacteristic of the measurement system disclosed that the movingparticle is imaged onto the lenticular grating. Thus, the distancebetween the lenticular grating and the moving particle has to beaccurately controlled. Furthermore, a set of lenses is placed behind thelens array in order to adequately collect light onto the detectors. Thiseliminates the possibility for realizing a compact one-element opticalsystem.

SUMMARY OF THE INVENTION

Thus, there is a need for a more compact and inexpensive displacementsensor, and a sensor that does not require accurate control of thedistance to the object.

According to the present invention the above-mentioned and other objectsare fulfilled by an optical displacement sensor system for detection ofdisplacement of an object, comprising a coherent light source forillumination of at least part of the object with spatially coherentlight, an optical member with at least three optical elements formapping of different specific first areas in space onto substantiallythe same second area in space thereby generating an oscillating opticalsignal caused by phase variations of light emanating from the objectmoving in the first areas.

It is a fundamental aspect of the present invention that a repetitiveoptical structure, i.e. the optical member, is utilized for formation ofa repetitive optical signal emitted by an illuminated moving object by

-   1) Illuminating the repetitive optical structure by the light source    for formation of a fringe pattern (similar to Laser Doppler    Anemometry), and/or,-   2) Illuminating an object by the light source and diverting light    from the illuminated object onto light sensors by the repetitive    optical structure.

The light source illuminates the object whereby a speckle pattern isformed on the object. The speckle pattern moves with movement of theobject, and speckle pattern movement is determined without a need forimaging the object onto the repetitive optical structure, i.e. theoptical member.

Since the speckle pattern is not imaged onto the optical member, thedistance and possible distance changes between the object and theoptical member substantially do not affect system performance.

In an embodiment of the invention well suited for Laser DopplerAnemometry applications, a collimated light source is provided forilluminating the optical member, and an imaging system is provided forimaging the optical member onto a measurement volume thereby forming afringe pattern in the measurement volume.

In another embodiment of the invention light emanating from the objectare received at an input plane. The optical member directs lightemanating from different parts of the input plane in substantially thesame direction by corresponding elements of the optical member.

In yet another embodiment of the invention, the above-mentionedembodiments are combined, i.e. the optical member is utilized both fortransmission of coherent light towards the object and for reception oflight emanating from the object.

Dividing the light beam from the light source into a plurality of lightbeams by illumination of the optical member increases safety, since thepower of an individual light beam that accidentally may enter a humaneye is lowered.

Further the signal to noise ratio is increased, since the projection ofa fringe-like pattern onto the object will make the speckle spectrumcorrespond to the optimum for the optical member.

The individual optical elements may interact with light by reflection,refraction, scattering, diffraction, etc, either alone or in anycombination, of light incident upon them. Thus, the individual opticalelements may be lenses, such as cylindrical lenses, spherical lenses,Fresnel lenses, ball lenses, etc, prisms, prism stubs, mirrors, liquidcrystals, etc.

Alternatively, the optical member may be formed by a diffractive opticalelement, such as holographically produced lenses, etc.

Further, the optical member may comprise a linear phase grating with asinusoidal modulation of the film thickness, e.g. in a photo resistfilm.

After interaction with the individual optical elements, the light may betransmitted through the elements or be reflected from the elements, e.g.appropriately coated for reflection. The system also comprises adetector with at least one optical detector element for conversion oflight incident upon it into a corresponding electronic signal.

The detector is positioned in the propagation path of the lightemanating from the optical member. The invention is most easilyunderstood considering an embodiment wherein the optical member is arepetitive optical member constituted by a linear array of cylindricallenses. The focal length of the lenses may be positive or negative. Forsimplicity only positive lenses are depicted in the appended figures.The input plane is located in front of the array of lenses at a distanceequal to the focal length of the lenses and perpendicular to thedirection of propagation of the incoming light emanating from theobject.

The object has a surface of a size that allows formation of a specklepattern. Surface roughness of the object causes formation of the specklepattern since surface deviations modify the phase of various parts ofthe incident light differently. Preferably, at least a part of thesurface is illuminated by a laser, and a speckle pattern could bedetected at the input plane, e.g. by intensity measurements, caused byvariations of the electromagnetic field along the input plane. In thefollowing, such electromagnetic intensity variations are termed “specklevariations”. When the object is displaced, the speckle variations movealong the input plane with a velocity proportional to the surfacevelocity or the rotational speed. The individual optical elements directthe light towards an optical detector of the embodiment. When a specklevariation at the input plane has moved a distance that is equal to thewidth of an individual optical element, the corresponding lightemanating from the individual optical element sweep across the area ofthe optical detector. This is repeated for the next optical element, andit is seen that when a speckle variation has traversed a distance equalto the length of the linear array, the optical detector is sweptrepetitively a number of times equal to the number of individual opticalelements of the linear array. It is seen that for a regular specklevariation pattern at the input plane with a speckle size that issubstantially equal to the size of an individual optical element, theintensity of the electromagnetic field at a detector element variesbetween a high intensity when light areas of the speckle variations arealigned with the optical elements and a low intensity when dark areas ofthe speckle variations are aligned with the optical elements, and thatthe frequency of the oscillations corresponds to the velocity ofdisplacement of the speckle variations in the direction of thelongitudinal extension of the linear array divided by the array pitch,i.e. the distance between individual neighboring optical elements.

This principle of operation applies in general to other embodiments ofthe present invention regardless of the type of optical member utilizedand regardless of whether or not an image of the object is formed at theinput plane. For example, a point source positioned substantially at theinput plane and emitting a diverging beam of light may illuminate theobject. The radiation is diffusely reflected by the object and receivedat the input plane. It is well known in the art that speckle variationdisplacements at the input plane is twice the correspondingdisplacements at the surface of the object regardless of the distancebetween the object and the input plane.

Alternatively, the object may be illuminated by a collimated beam oflight in which case, the displacement of speckle variations at the inputplane is equal to corresponding displacements at the surface of theobject independent of the distance between the object and the inputplane.

In still another embodiment of the present invention, a Fouriertransforming lens is positioned between the object and the input planein such a way that the input plane is positioned at the Fourier plane ofthe Fourier transforming lens, i.e. at the back focal plane of the lens,whereby rotational displacement of the object can be determinedindependent of the distance between the object and the input plane.Furthermore, the detected frequency will be independent of the radius ofcurvature of the rotating object and independent of the wavelength.Besides, a transverse displacement of the object will only give rise tospeckle de-correlation and not give rise to speckle displacement.

The optical member provides a spatial filtering of the electromagneticfield at the input plane in such a way that moving speckle variations ofa size comparable to the size of the individual optical elements leadsto an oscillating detector signal. Speed of movement can e.g. bemeasured by zero-crossing detection of this signal.

It is seen that two-dimensional speckle displacement may be determinedwith an embodiment of the present invention having a two-dimensionalarray of optical elements.

In a preferred embodiment of the present invention, the system furthercomprises an imaging system for imaging part of the input plane onto theat least one detector element whereby each of the individual opticalelements in combination with the imaging system images specific parts ofthe input plane onto the same specific area of an output plane so thatpoints at the input plane that are positioned at the same relativepositions in relation to adjacent respective optical elements are imagedonto the same point at the optical detector. As further explained below,without the imaging system, there is a small distance between imagedpoints at the detector for corresponding points at the input planehaving the same relative position in relation to respective opticalelements. However, the accuracy of the system may still be sufficientand will depend on the actual size of the system.

The optical member and the above-mentioned imaging system may be mergedinto a single physical component, such as a molded plastic component, inorder to obtain a further compact system suited for mass production.

The displacement sensor with the optical member facilitatesdetermination of speckle variation movement at the input plane in adirection of the optical member. An optical system positioned betweenthe object and the input plane determines the type of object movementthat is determined by converting the speckle variations arising from thespecified object movement into a linear speckle translation in thesystem input plane. The object movement could be in-plane orout-of-plane rotation and/or displacement, such as one- ortwo-dimensional displacement, one- or two-dimensional velocity, angulardisplacement, angular velocity, etc.

The number, size, and position of the detector elements together withthe processing of the signals obtained from the elements, such assubtraction, addition, etc, determine the suppression of harmonics inthe output signal. Specifically, the direction of movement of specklevariations can be probed in case a quadrature (or close to quadrature)signal is obtained.

It is an important advantage of the present invention that thedisplacement sensor only comprises a few optical detector elements, suchas one, two, four, six or seven, etc, elements.

Occurrence of velocity signal drop out may be reduced by provision of asecond set of optical detector elements that is displaced in relation tothe existing set of detector elements so that a signal that isstatistically independent of the other signal may be available from oneset of detector elements during absence of a signal from the other setof detector elements. Thus by proper processing of the two signals, e.g.switching to a set of detector elements generating a velocity signal,occurrence of signal drop out is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention reference will nowbe made, by way of example, to the accompanying drawings, in which:

FIG. 1 schematically illustrates a preferred embodiment of adisplacement sensor according to the present invention,

FIG. 2 illustrates the operating principle of the displacement sensor ofFIG. 1,

FIG. 3 schematically illustrates another preferred embodiment of adisplacement sensor according to the present invention,

FIG. 4 illustrates the operating principle of the displacement sensor ofFIG. 3,

FIG. 5 schematically illustrates electromagnetic wave propagation in thedisplacement sensor of FIG. 1,

FIG. 6 schematically illustrates a displacement sensor with a Fresnellens array,

FIG. 7 schematically illustrates a displacement sensor with a linearphase grating having a sinusoidal modulation in the film thickness,

FIG. 8 schematically illustrates a displacement sensor with a prismarray,

FIG. 9 illustrates the operating principle of the displacement sensor ofFIG. 8,

FIG. 10 schematically illustrates a displacement sensor with an array ofprism stubs,

FIG. 11 illustrates the operating principle of the displacement sensorof FIG. 10,

FIG. 12 illustrates an embodiment operating like a Laser DopplerAnemometer,

FIG. 13 illustrates another embodiment operating like a Laser DopplerAnemometer,

FIG. 14 illustrates an embodiment suited for determination of thevelocity of a particle, where direction for particle velocity can bedetermined.,

FIG. 15 is a plot of a detector element signal from an embodiment shownin FIG. 12 or 13,

FIG. 16 is a plot of a signal from a displaced optical detector elementhaving a phase lag in relation to the signal shown in FIG. 14,

FIG. 17 is a plot of the difference between the signals shown in FIGS.14 and 15,

FIG. 18 illustrates the definition of the input plane for an embodimentof the invention,

FIG. 19 illustrates the definition of the input plane for anotherembodiment of the invention,

FIG. 20 illustrates the definition of the input plane for yet anotherembodiment of the invention,

FIG. 21 illustrates combining of optical components of a displacementsensor according to the present invention,

FIG. 22 illustrates another way of combining of optical components of adisplacement sensor according to the present invention,

FIG. 23 illustrates combining of optical components of a displacementsensor with a prism array according to the present invention,

FIG. 24 schematically illustrates a displacement sensor according to thepresent invention with optical components combined with a prism,

FIG. 25 schematically illustrates another displacement sensor accordingto the present invention with optical components combined with a prism,

FIG. 26 schematically illustrates a two-dimensional array of prismstubs,

FIG. 27 illustrates electromagnetic wave propagation of waves to thedetector plane having been refracted by a prism stub,

FIG. 28 schematically illustrates the operation of a linear displacementsensor system according to the present invention,

FIG. 29 schematically illustrates the operation of another lineardisplacement sensor system according to the present invention,

FIG. 30 schematically illustrates the operation of a displacement sensorsystem for determination of rotational displacement according to thepresent invention,

FIG. 31 schematically illustrates a reflection configuration of thepresent invention for determination of linear displacement,

FIG. 32 schematically illustrates a reflection configuration of thepresent invention for determination of rotational displacement,

FIG. 33 schematically illustrates a reflection configuration of thepresent invention for determination of 2D rotational displacement,

FIG. 34 schematically illustrates a reflection configuration of thepresent invention for determination of 2D linear displacement,

FIG. 35 schematically illustrates an embodiment with three lenticulararrays,

FIG. 36 schematically illustrates an embodiment with four lenticulararrays,

FIG. 37 schematically illustrates a simple detector configuration,

FIG. 38 is a plot of a signal provided by the detector illustrated inFIG. 37,

FIG. 39 is a plot of the power spectrum of the signal shown in FIG. 38,

FIG. 40 schematically illustrates another detector configuration,

FIG. 41 is a plot of a signal provided by the detector illustrated inFIG. 34,

FIG. 42 is a plot of the power spectrum of the signal shown in FIG. 41,

FIG. 43 schematically illustrates yet another detector configuration,

FIG. 44 is a plot of a signal provided by the detector configurationillustrated in FIG. 43,

FIG. 45 is a phase plot of the signal shown in FIG. 44,

FIG. 46 schematically illustrates still another detector configuration,

FIG. 47 is a plot of a signal provided by the detector illustrated inFIG. 46,

FIG. 48 is a phase plot of the signal shown in FIG. 47,

FIG. 49 schematically illustrates a miniaturized embodiment of theinvention,

FIG. 50 schematically illustrates another miniaturized embodiment of theinvention,

FIG. 51 schematically illustrates an embodiment with common transmittingand receiving optics,

FIG. 52 schematically illustrates the operating principle of theembodiment shown in FIG. 51,

FIG. 53 illustrates the intensity distribution at the surface of anobject illuminated by the embodiment shown in FIG. 51, and

FIG. 54 illustrates (unintentional) illumination of the human eye by theembodiment shown in FIG. 51.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically shows a displacement sensor 10 according to thepresent invention with a linear array 12 of cylindrical lenses 18. f₁ isthe focal length of the cylindrical lenses 18. The input plane 14 islocated at a distance equal to the focal length f₁ of the lenses 18 andperpendicular to the direction 16 of propagation of the incoming lightemanating from the object (not shown). In this embodiment 10, a part ofthe surface of the object (not shown) scatters light onto the inputplane 14. Preferably, a laser illuminates the part of the surface, and aspeckle variation is generated at the input plane. When the object isdisplaced, the speckle variations move correspondingly along the inputplane 14. The individual cylindrical lenses 18 direct the light 16, 20towards a refractive lens 22 having a focal length f₂ and beingpositioned a distance equal to f₁+f₂ from the linear array 12. The lens22 further refracts the light 20 into waves 24 propagating towardsoptical detector elements 26, 28, and 30 positioned at the focal planeof lens 22. In this way, the input plane 14 is repeatedly imaged onto anoutput plane 15. The detector elements 26, 28, 30 are positioned so thattheir individual surfaces for reception of light coincide with theoutput plane 15. It is seen that an area 32 of the input plane is imagedonto an area 34 of a detector element 28 and that corresponding areas 36that are located at the same relative positions in relation to adjacentrespective cylindrical lenses 18 are imaged onto the same area 34 of theoptical detector constituted by the detector elements 26, 28, 30.

It should be noted that the distance between the linear array 12 and thelens 22 is chosen to be equal to f₁+f₂ in the present example for easeof explanation of the operation of the displacement sensor 10. However,the displacement sensor 10 operates with any distance between the lineararray 12 and the lens 22. For compactness it may be preferred to set thedistance to zero.

It should also be noted that in an imaging system as the one shown inFIG. 28, a rotational displacement of the object does not lead totranslation of speckle variations in the input plane. However,rotational displacement of the object will typically lead to speckleboiling.

The operating principle of the displacement sensor of FIG. 1 is furtherillustrated in FIG. 2. When a speckle variation 16 at the input plane 14has moved a distance 38 that is equal to the width Λ₀, i.e. the pitch,of an individual optical element 18, the corresponding image formed bythe combination of lens 22 and the respective cylindrical lens 18 sweepsacross the area 40 of the optical detector elements 26, 28, and 30. Thisis repeated for the other optical elements 18, and it is seen that whena speckle variation has traversed a distance equal to the length of thelinear array 12, the optical detector 26, 28, 30 is swept repetitively anumber of times equal to the number of individual optical elements 18 ofthe linear array 12. It is seen that for a regular speckle variationpattern at the input plane with an average speckle size that iscomparable with the size 38 of an individual optical element 18, theintensity of the electromagnetic field at a detector element 26, 28, 30varies between a high intensity when bright areas of the specklevariations are aligned with the sensor elements 26, 28, 30 and a lowintensity when dark areas of the speckle variations are aligned with theoptical elements 26, 28, 30, and that the frequency of the oscillationscorresponds to the velocity of displacement of the speckle variations inthe direction Δx of the longitudinal extension of the linear array 12divided by the array pitch, i.e. the distance between individualneighboring optical elements.

As previously mentioned, the same principle of operation applies ingeneral to other embodiments of the present invention regardless of thetype of optical member utilized and regardless of whether or not animage of the object is formed at the input plane 14.

With reference again to FIGS. 1 and 2 wherein Λ₀ denotes the pitch 38 ofthe optical member 12 and f₁ is the focal length of the individualoptical elements 18 of the optical member 12 and f₂ is the focal lengthof the lens 22, the distance D₀ in the detector plane 26 and 30,repetitively swept by a speckle traversing the input plane is given by$D_{0} = {\Lambda_{0}{\frac{f_{2}}{f_{1}}.}}$

In the detected electrical signal D₀ corresponds to the period of thesignal, i.e. a 360° phase shift. This equation is valid for any distancebetween the optical member 12 and the lens 22.

For the aperture of the system D, e.g. in FIG. 1D is equal to thediameter of the lens 22, the following equation should preferably befulfilled: $\frac{D}{f_{2}} \geq {\frac{\Lambda_{0}}{f_{1}}.}$

Further, it is preferred that the effect of the individual opticalelements is governed by the lens effect and not by diffraction, i.e.:$\frac{f_{1}}{k\quad\Lambda_{0}^{2}}{\operatorname{<<}1.}$

Where k is the optical wavenumber. FIG. 3 schematically shows anotherdisplacement sensor 11 according to the present invention without thelens 22, i.e. without an imaging system that images the input plane 14onto the output plane 15. In this embodiment, the receiving areas of thedetector elements 26, 28, 30 define the output plane 15. As more clearlyillustrated in FIG. 4, without the imaging system, there is a smalldistance between imaged points 34 at the detector 26, 28, 30 forcorresponding points 32 at the input plane 14 having the same relativeposition in relation to respective optical elements 18. However, theaccuracy of the system 11 may still be sufficient and will depend on theactual size of the system 11.

In FIG. 5, the image forming of the displacement sensor shown in FIG. 1is further illustrated, and it is seen that the position of theintersection 34 between the optical detector element 28 and incidentlight 24 is independent of the slope of the electromagnetic wave 16incident on the input plane 14. It only depends on the relative positionof the intersection 32 between the input plane 14 and the input wave 16in relation to the adjacent optical element 18.

FIG. 6 schematically illustrates a displacement sensor similar to thesensor shown in FIG. 1, wherein the linear array of cylindrical lenseshas been substituted with a Fresnel lens array 42. The linear array ofcylindrical lenses may alternatively be substituted by a diffractiveoptical element 42.

Likewise FIG. 7 schematically illustrates a displacement sensor similarto the sensor shown in FIG. 1, wherein the linear array of cylindricallenses has been substituted with a linear phase grating 43 with asinusoidal modulation of the film thickness, e.g. in a photo resistfilm. The phase grating can be made by exposing a (thick) photo resistplate with an interference pattern made by crossing two laser beams.Incident light will primarily be diffracted in the “plus first” and“minus fist” order. Besides, non-diffracted light will pass directlythrough the phase grating.

In FIG. 8, an alternative embodiment of the displacement sensor shown inFIG. 3 is illustrated, wherein the linear array of cylindrical lenseshas been substituted by a linear array 44 of prisms. The two sides ofeach prism refract incoming rays of light towards two respectivedetector elements 26, 30.

The operating principle of the displacement sensor of FIG. 8 is moreclearly illustrated in FIG. 9 showing that a speckle variation isalternatingly directed toward the two respective detector elements 26,30 when the speckle variation traverses the linear prism array 44 alongits longitudinal extension.

In FIG. 10, a similar embodiment of the displacement sensor isillustrated, wherein the linear array of cylindrical lenses has beensubstituted by a linear array 44 of prism stubs. As before, the twosides of each prism refract incoming rays of light towards tworespective detector elements 26, 30 while the top surface transmits orrefracts incoming rays toward a third detector element 28.

The operating principle of the displacement sensor of FIG. 10 is moreclearly illustrated in FIG. 11 showing that a speckle variation isalternatingly directed toward the three respective detector elements 26,28, 30 when the speckle variation traverses the linear prism stub array44 along its longitudinal extension.

It should be noted that the direction of propagation of waves refractedby the prisms depends on the slope of the direction of propagation ofthe incoming light. It should also be noted that the phase differencebetween detector signals is fixed. It is an important advantage of theseembodiments of the invention that the phase difference is determined bythe geometry of the optical member and independent on the detectorelement positions. It is another advantage that utilization of prismsfacilitates utilization of small detectors.

FIG. 12 illustrates an embodiment 80 of the invention that operates likea Laser Doppler Anemometer. Light 82 emitted from a point source laser84 is collimated by lens 86 and the collimated light illuminates anarray 12 of cylindrical lenses. Lenses 88 and 90 form a telescope thatimages the focal spots of the cylindrical lenses 12 in the measuringvolume 92 whereby a set of straight and equidistant fringe planes 94 areformed in the measurement volume 92. It is seen that the first areasoccupied by the fringes 94 are mapped into the same second area 96 atthe laser source 84.

FIG. 13 illustrates another embodiment 100 of the invention that issimilar to the embodiment 80 shown in FIG. 12, however, in thisembodiment an array 12 of spherical lenses has substituted the array ofcylinder lenses in FIG. 12. This leads to equidistant and straightcylindrical focal lines 94.

Particles or a solid surface passing the fringes created in themeasuring volume will scatter light. A detector place at an arbitraryposition will give rise to a modulated signal, the frequency of which isgiven by the velocity component perpendicular to the fringes divided bythe fringe distance.

FIG. 14 illustrates the embodiment 102 also shown in FIG. 1 formeasurement of particle velocity 106. A laser beam 104 is focused onto ameasurement volume 108 that is imaged onto the input plane 15 of thesystem with lens f₁. When a particle 110 traverses the measurementvolume 108, its image 112 will traverse the array 12 of cylindricallenses, and each of the detectors 26, 28, 30 receives an oscillatingoptical signal similar to the signal created by fringes in a LaserDoppler Anemometer. Thus, each detector element 26, 28, 30 receiveslight from the particle 110 as if the particle 110 traverses a set offringes, in the following denoted virtual fringes. FIG. 15 is a plot ofthe output signal 114 from one of the detector elements generated inresponse to the oscillating optical signal. A corresponding signal 116from an adjacent detector element is shown in FIG. 16. This signal 116is phase shifted in relation to the signal 114 shown in FIG. 15 becauseof the physical displacement of the detector elements.

Since the low frequency pedestals of the two signals are substantiallyidentical, the difference between the two signals 114, 116 is anAC-signal 118 as shown in FIG. 17. It is preferred that the phase shiftbetween the two detector elements 26, 28; 28, 30 is substantially equalto 90° so that the direction of the particle velocity can be deducedwhereby the requirement of a costly Bragg-cell is eliminated. The angleof the incident laser beam is not critical. A forward scattering systemprovides the largest signals, however, a back scattering system may bepreferred for other reasons.

In this embodiment 102 of the invention, it is seen that first areas ofthe measurement volume 108, i.e. corresponding to the virtual fringes,are mapped into the same second area at a detector element.

FIG. 18 illustrates the definition of the input plane 14 of the system.In the embodiment 10 shown in FIG. 1, the output plane 15, defined bythe detector element 26, 28, 30 surfaces, is imaged onto the input plane14 by the combination of the lens array 12 and the lens 22. Thus, rays120 emitted from the output plane 15 would be focused onto the inputplane 14 by an element 18 of the optical member 12.

FIG. 19 illustrates the definition of the input plane 14 for theembodiment 11 shown in FIG. 3. There is no imaging system in thisembodiment 11, however still, rays 120 emitted from the output plane 15would be focused onto the input plane 14 by an element 18 of the opticalmember.

FIG. 20 illustrates the definition of the input plane for the embodimentshown in FIG. 8. Also in this embodiment, there is no imaging system,however still, rays 120 emitted from the output plane 15 would befocused onto the input plane 14 by an element of the optical member 44.

FIGS. 21-23 illustrates various ways of combining the optical componentsof the previously illustrated displacement sensors in order to provide afurther compact system suited for mass production.

In FIGS. 24 and 38, the optical components have been combined with aprism to limit the linear extension of the system. In FIG. 25, the lens22 shown in FIG. 13 has been substituted by a concave mirror 23.

FIG. 26 schematically illustrates a repetitive optical member in theform of a two-dimensional array 46 of prism stubs for determination ofspeckle displacement in two dimensions. FIG. 27 illustrateselectromagnetic wave propagation of waves 50 having been refracted by aprism stub. It is seen that a flat top prism refracts incoming beams oflight into five directions towards five different respective detectorelements. The phase difference between detector element output signalsdepends solely on the geometry of the flat top prism array. It does notdepend on detector element position. A prism array 44, 46, 48facilitates the use of small detector elements.

FIG. 28 schematically illustrates a displacement sensor system 52according to the present invention, comprising an imaging system 54 thatimages a part 56 of the surface of a moving object 58 onto the inputplane 14 of a displacement sensor 10 also shown in FIG. 1. The object 58is illuminated with a collimated laser beam 60 so that the velocitycomponent along the intersection of the input plane 14 and the plane ofFIG. 28 determined at the input plane 14 is the magnification ratio ofthe imaging system 54 times the corresponding velocity component of thesurface 56.

FIG. 29 schematically illustrates a displacement sensor system 62 thatdiffers from the displacement sensor system shown in FIG. 28 in that thesystem 62 does not have an imaging system 54 and that the object 58 isilluminated with a divergent laser beam 64 emanating from a pointsource, e.g. a VCSEL, positioned at the input plane 14. It is well knownin the art that speckle variation displacements at the input plane istwice the corresponding displacements at the surface of the objectregardless of the distance between the object and the input plane. Thus,the velocity component along the intersection of the input plane 14 andthe plane of FIG. 29 determined at the input plane 14 is two times thecorresponding velocity component of the surface 56.

FIG. 30 schematically illustrates yet another displacement sensor system66 according to the present invention, comprising a Fourier transforminglens 70 positioned so that its Fourier plane, i.e. the back focal planeof lens 70, coincides with the input plane 14 of a displacement sensor10 also shown in FIG. 1. The object 68 is illuminated with a collimatedlaser beam 60 and the velocity component of the speckles in the inputplane along the intersection of the input plane 14 and the plane of FIG.18 corresponds to the angular velocity of the object 68.

It should be noted that translational displacement of the object 68 doesnot lead to translation of speckle variations in the input plane.However, translational displacement of the object 68 will typically leadto speckle boiling.

FIG. 31 schematically illustrates a displacement sensor system 75 fordetermination of linear displacement with a reflecting member 12comprising a linear array of cylindrical concave mirrors 18 performingan optical function similar to the optical function of the cylindricallenses shown in e.g. FIG. 1. The displacement sensor operates similar tothe operation of the sensor shown in FIG. 3 apart from the fact that theoptical member 12 shown in FIG. 21 reflects light while thecorresponding optical member 12 shown in FIG. 3 refracts light.

FIG. 32 schematically illustrates another reflection configuration 76 ofthe present invention for determination of rotational displacement alsohaving a reflecting member 12 comprising a linear array of cylindricalconcave mirrors 18. The system illustrated operates similar to thesystem illustrated in FIG. 30, however, it should be noted that in thesystem 76, the functions of lenses 22 and 70 of the system illustratedin FIG. 30 are combined into one lens 22.

FIG. 33 schematically illustrates a displacement sensor system 77 fordetermination of rotation in two dimensions. The system 77 is similar tothe one-dimensional system 76 illustrated in FIG. 32, however, thelinear array of cylindrical concave mirrors has been replaced by atwo-dimensional array 12 of spherical concave mirrors 18, and twooptical detector elements 27, 29 have been added facilitating detection,in combination with detector element 28, of speckle movement in adirection substantially perpendicular to the direction of movementdetected by the combination of optical detector elements 26, 28, 30.

Likewise, FIG. 34 schematically illustrates a displacement sensor system78 for determination of displacement in two dimensions. The system 77 issimilar to the one-dimensional system 75 illustrated in FIG. 31,however, the linear array of cylindrical concave mirrors has beenreplaced by a two-dimensional array 12 of spherical concave mirrors 18,and two optical detector elements 27, 29 have been added facilitatingdetection, in combination with detector element 28, of speckle movementin a direction substantially perpendicular to the direction of movementdetected by the combination of optical detector elements 26, 28, 30.

FIG. 35 is a front view of a displacement sensor system 130 fordetermination of displacement in two dimensions and in-plane rotation,simultaneously. The system 130 comprises a VCSEL 132 positioned behind acollimating lens 134 whereby the object is illuminated by a collimatedlight beam. Three lenticular cylindrical lens arrays 136, 138, 140 arepositioned in a common plane with a mutual angular separation ofapproximately 120° for determination of velocity components indicated bythe respective arrows 142, 144, 146. Detectors 148, 150, 152 arepositioned behind the respective lenticular arrays for conversion ofreceived light to an electrical signal.

Velocity components V_(x) and V_(y) are defined by the co-ordinatesystem 154. V_(x) and V_(y) and rotational velocity V_(φ), arecalculated according to the equations 156.

FIG. 36 is a front view of a displacement sensor system 160 fordetermination of displacement in two dimensions and in-plane rotation,simultaneously. The system 160 comprises a VCSEL 162 in the drawingpositioned behind a collimating lens 164 whereby the object isilluminated by a collimated light beam. Four lenticular cylindrical lensarrays 166, 168, 170, 172 are positioned in a common plane with a mutualangular separation of approximately 90° for determination of velocitycomponents indicated by the respective arrows 174, 176, 178, 180.Detectors 182, 184, 186, 188 are positioned behind the respectivelenticular arrays for conversion of received light to an electricalsignal. Velocity components V_(x) and V_(y) and rotational velocityV_(φ) are calculated according to the equations 190.

FIG. 37 shows a fundamental detector element configuration 80 of anembodiment of the invention. FIG. 38 is a plot of the detector signal 82and FIG. 39 is a plot of the power spectrum 84 of the detector signal.It should be noted that the low frequency part 86 and the secondharmonic 88 of the spectrum 84 are quite significant. The low frequencynoise leads to a variation of the running mean value which willintroduce significant errors in velocity determinations based onzero-crossing detection. The width of the detector has been selected foroptimum suppression of the third harmonic of the fundamental frequency.The detector element is assumed to have a rectangular shape and thus,the power spectrum of the detector function is a sinc-function. In orderto eliminate every third harmonic of the detector output signal, thewidth of each detector element is selected to be substantially equal toone third of the full width of the detector array that is selected to beequal to the width repetitively swept by a speckle traversing the inputplane. In FIG. 34, a configuration of two matched detector elements 92,94 is shown. The distance between the elements corresponds to a phaseshift of 180°. The output signals from the detector elements aresubtracted for suppression of the low frequency part of the signals andthe even harmonic frequencies of the fundamental frequency. Thedifference signal 96 is plotted in FIG. 41, and the power spectrum 98 isplotted in FIG. 42. The suppression of the low frequency part 86 and thesecond harmonic 88 is clearly demonstrated by comparison with FIG. 39.

An almost-phase-quadrature detector configuration 100 is shown in FIG.43, wherein six detectors of equal size form two subtracted signals 102,104. The two subtracted signals 102, 104 are 60 degrees out of phase,and therefore suitable for determination of e.g. direction of thespeckle translation or sub-radian phase resolution. In thisconfiguration 100, an exact phase quadrature can not be achieved withoutchanging the detector width 106 thereby reducing the suppression of thethird harmonic. The subtracted almost-phase-quadrature signals 108, 110are plotted in FIG. 44, and FIG. 45 is a phase plot 112 of the signals108, 110. The phase plot 112 has an elliptical shape which facilitatesdetermination of the direction of the speckle translation and eventuallysub-radian measurement accuracy. However, due to the elliptically shapeof the phase plot this configuration will be noise sensitive.

The detector configuration 114 shown in FIG. 46 provides a substantiallyexact phase-quadrature detector arrangement. Seven detectors of threedifferent sizes form two subtracted signals 116, 118. The two signals116, 118 are 90 degrees out of phase and therefore facilitatedetermination of direction of object velocity and sub-radian phaseresolution. FIG. 47 is a plot of the subtracted phase-quadrature signals120, 122, and FIG. 48 is the corresponding phase plot 124. The phaseplot 124 is circular facilitating determination of the direction of theobject velocity and sub-radian measurement accuracy. The circular shapeof the traces in the phase plot makes this configuration less sensitiveto noise.

FIGS. 49 and 50 show preferred miniaturized embodiments of the presentinvention. The working distance between the input plane and the targetsurface is 40 mm. The pitch of the cylindrical lens array is 30 μm andthe focal length is 38.7 μm. The distance between detector elements 26,28 is 1.6 mm, and the distance between the lens 22 and the detectorelements 26, 28 is 3.2 mm.

In the following and with reference to FIGS. 51-54, an embodiment 200 ofthe invention is disclosed, wherein the optical member 12 is a cylinderlens array 12 and is utilized both for transmission of coherent lighttowards the object 204 and for reception of light emanating from theobject 204. In the embodiment, the light beam emitted from the VCSEL 202is collimated by the lens 22 and divided into a plurality of beams bythe cylinder lens array 12 for illumination of the object 204. Thisleads to two significant advantages: 1) The signal caused by thespeckles are modulated by the plurality of beams whereby the specklespectrum is concentrated in a frequency range that is optimized for thesystem. 2) Safety classification of the system will be based on thepower in each of the individual beams.

In many applications direct access to the emitted radiation may pose aproblem since the wavelength is in the near infrared region (app. 850nm), where the safety regulations are most severe. This is due to thefact that the eye is able to focus the radiation on the retina, yet thesensitivity of the eye is extremely low in this wavelength region. Thismeans that a damaging radiation could occur without proper warning fromthe sensory system, i.e. the visual impact. Therefore this issue is ofvital importance for the user of laser-based systems, especially inconsumer products.

The laser safety standard: “Safety of laser products, Part 1, IEC60825-1, Ed. 1.1, 1998-01 describes the main safety hazard for thevisible and near infrared region to retinal damage. A maximumpermissible emission level (AEL) is assigned for a particular class oflaser products. The lowest class for laser products is Class 1. It isdesired that laser-based products for the consumer market fall withinclass 1.

For a 850 nm collimated light beam having a diameter of 1 mm, the AEL is0.24 mW below which no retinal damage is foreseen. However for mostapplications this is insufficient for generation of acceptable signals.

However, with the illustrated embodiment, a VCSEL 202 emits a 850 nmlight beam with a diameter of 0.4 mm. The diffraction-limited spot atthe retina is app. 0.12 mm provided that the diameter of the eye is app.50 mm. When collimated light beams are emitted, as schematicallyillustrated in FIG. 54, with an angular separation of app. 3°, separatespots are formed on the retina with a mutual distance of app. 2.5 mm.There is substantially no spot overlap, and therefore the AEL maximumrequirement should be fulfilled for each of the individual spots.However, the total power emitted by the VCSEL may be several times theAEL value.

By illumination of the object with a plurality of beams, the specklesare modulated with a periodic structure that matches the periodicstructure of the lens array. This causes a Moiré-like effect with anexpected stronger signal as a result. The embodiment is schematicallyillustrated in FIG. 51. The intensity distribution at the illuminatedobject 204 is illustrated in FIG. 53 with the following parameters:

-   A_(s) is the aperture diameter at the lens array position-   w_(d) is the distance to the object-   λ is the wavelength of the emitted light-   Λ₀ is the width of the individual lenses in the array, i.e. the    pitch of the array.-   f_(c) is the focal length of the lens array.

For an embodiment with A_(s)=1 mm, w_(d)=3 mm, λ=1 μm, Λ₀=15 μm, andf_(c)=30 μm, the horizontal spot separation is 200 μm and the spotheight is 1.5 mm. The speckle spectrum at the lens array will exhibit apeak at a spectral position given by the product of the wavelength andthe distance from the object to the lens array divided by the spotseparation as given by the expression in FIG. 53.

For determination of two-dimensional movement two perpendicularlyoverlapping lens arrays may be provided in the light path.

1-32. (canceled)
 33. An optical displacement sensor system for detectionof displacement of an object, comprising: a coherent light source forillumination of at least part of the object with spatially coherentlight, an optical member with at least three optical elements formapping of different specific first areas in space onto substantiallythe same second area in space thereby generating an oscillating opticalsignal caused by phase variations of light emanating from the object,and an optical detector for conversion of the oscillating optical signalinto an oscillating electronic signal having a frequency correspondingto the velocity of the object.
 34. A system according to claim 33,wherein the optical member is a repetitive optical member comprisingsubstantially identical optical
 35. A system according to claim 33,wherein the detector comprises a plurality of optical detector elementsfor conversion of the optical signal into a corresponding electronicsignal.
 36. A system according to claim 33, wherein the optical elementsare lenses.
 37. A system according to claim 36, wherein the opticalelements are cylindrical lenses.
 38. A system according to claim 36,wherein the optical elements are spherical lenses.
 39. A systemaccording to claim 36, wherein the optical elements are Fresnel lenses.40. A system according to claim 36, wherein the optical elements areformed by a linear phase grating with a sinusoidal modulation ofsubstrate thickness.
 41. A system according to claim 33, wherein theoptical elements are prisms.
 42. A system according to claim 33, whereinthe optical elements are prism stubs.
 43. A system according to claim33, wherein the optical member is a diffractive optical element.
 44. Asystem according to claim 33, wherein the optical elements reflectlight.
 45. A system according to claim 33, wherein the optical member isa linear array of optical elements.
 46. A system according to claim 33,wherein the optical member is a two-dimensional array.
 47. A systemaccording to claim 33, wherein a laser illuminates the object.
 48. Asystem according to claim 35, comprising an input plane for reception oflight emanating from the object, and wherein the optical member isadapted to direct light emanating from different parts of the inputplane in substantially the same direction by corresponding elements ofthe optical member.
 49. A system according to claim 48, furthercomprising a system for mapping light with a specific angle of incidenceonto a corresponding specific location at the at least one opticaldetector element.
 50. A system according to claim 49, wherein theoptical member and the system for mapping are merged into a singlephysical component.
 51. A system according to claim 48, furthercomprising a Fourier transforming lens that is positioned between theobject and the input plane in such a way that the input plane ispositioned at the Fourier plane of the Fourier transforming lens wherebyrotational displacement of the object can be determined.
 52. A systemaccording to claim 48, wherein the object is illuminated by a divergentbeam of light emitted by a point source positioned substantially at theinput plane.
 53. A system according to claim 48, wherein a collimatedbeam of light illuminates the object.
 54. A system according to claim48, wherein the average size of speckles formed on the input plane issubstantially identical to the size of an individual optical element.55. A system according to claim 33, wherein the coherent light sourceilluminates the optical member for formation of a plurality of lightbeams illuminating the object.
 56. A system according to claim 33,further comprising a collimated light source for illuminating theoptical member.
 57. A system according to claim 56, further comprisingan imaging system for imaging the optical member onto a measurementvolume thereby forming a fringe pattern.
 58. A system according to claim33, further comprising a plurality of optical members positioned in asubstantially common plane with at least three optical elements formapping of different specific first areas in space onto substantiallythe same second area in space for determination of different velocitycomponents in the common plane.
 59. A system according to claim 35,wherein the width of the optical detector elements has been selected foroptimum suppression of the third harmonic of the fundamental frequencyof the detector signal.
 60. A system according to claim 35, wherein thedetector comprises a first set of two matched optical detector elementswith a mutual distance corresponding to a phase shift of substantially180° of the fundamental frequency.
 61. A system according to claim 60,wherein the detector further comprises a second set of two matchedoptical detector elements with a mutual distance corresponding to aphase shift of substantially 180°, the phase of the difference signalbetween the two sets having a mutual distance corresponding to a phaseshift of substantially 60°.
 62. A system according to claim 60, whereinthe detector further comprises a second set of two matched opticaldetector elements with a mutual distance corresponding to a phase shiftof substantially 180°, the phase of the difference signal between thetwo sets having a mutual distance corresponding to a phase shift ofsubstantially 90°.
 63. A system according to claim 35, furthercomprising a second set of optical detector element that is displaced inrelation to the at least one optical detector elements for provision ofan output signal that is statistically independent of the output signalof the at least one optical detector elements whereby the influence oflack of signal due to signal drop out may be minimized.